This invention relates to a spark ignition internal combustion engine operated on a lean air-fuel mixture supplemented with hydrogen gas, and to a method of operating the same.
As is well known in the art, internal combustion engines that are operated on over lean air-fuel mixtures emit low levels of nitrogen oxide (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC). However, internal combustion engines operated on the over lean mixtures (for example, air-to-fuel ratio; 18 or more: 1) usually misfire and therefore stable operation is not possible. The misfiring results from failure to ignite the lean mixture which has low fuel particle density and consequencial poor flame front movement in the combustion chamber after ignition.
To overcome the above mentioned drawback, operation of the engine on over lean mixtures supplemented with hydrogen gas has been proposed. In this operation, flammable hydrogen gas can be diffused among fuel particles in the combustion chambers and therefore misfiring is prevented and the flame front movement improved.
The prior art engine operation on the hydrogen-supplemented fuel has contributed to lowering the emission levels of NO.sub.x and CO, however, a problem has been encountered in that the air-fuel mixture adjacent the inner wall of the combustion chambers is not effectively burned and therefore relatively high levels of unburned hydrocarbons are emitted. This results from the fact that the temperature of the inner wall surface of the combustion chambers is not increased due to the low calorific value of the lean mixture, and release or absorption of heat generated by combustion through the walls of the combustion chambers.